It is known to provide a SAW device with tapered interdigital transducers (IDTs) in order to provide a changing SAW wavelength, and hence a changing center frequency for SAW propagation, across the aperture of the SAW device, thereby to provide the SAW device with a relatively broad fractional bandwidth. The taper may be linear, hyperbolic, or in accordance with some other function, and tapers can be applied to IDTs of different types, for example to bidirectional IDTs and to SPUDTs (single phase unidirectional trandsucers) with and without known weighting techniques such as withdrawal weighting, finger position or width weighting, etc.
Tapered IDTs and SAW devices using them have also been referred to as having slanted finger geometries, but the term "slanted" is not used further herein to avoid potential confusion with other and different types of SAW device, such as slanted array correlators (SACs).
For example, SAW devices using tapered IDTs in various configurations are known from U.S. Pat. No. 4,635,008 issued Jan. 6, 1987 and entitled "Dispersive SAW Filter With Tapered Transducers", U.S. Pat. No. 4,746,882 issued May 24, 1988 and entitled "SAW Multiplexer Using Tapered Transducers", U.S. Pat. No. 4,908,542 issued Mar. 13, 1990 and entitled "SAW Tapered Transducers", and U.S. Pat. No. 5,831,492 issued Nov. 3, 1998 and entitled "Weighted Tapered SPUDT SAW Device", all in the name of Leland P. Solie.
It is also known in satellite communications systems to use SAW device modules providing a switched or programmable bandwidth. For example, "SAW Based Systems for Communications Satellites" by R. C. Peach, 1995 IEEE Ultrasonics Symposium, pages 159-166 describes a programmable SAW filter module with four SAW filters having different bandwidths which are switched singly or in groups to provide a desired bandwidth, and "SAW Filters and Resonators for Public Communications Systems" by Y. Yamamoto, 1993 Ultrasonics Symposium, pages 95-103 describes SAW transversal filters having pass bands adjacent one another for different channels to provide for changing filter bandwidth digitally. In order to provide the desired bandwidths with necessary accuracy, for example to provide amplitude cross-overs at -6 dB with continuous phase responses for adjacent channel filters as described in the latter article, such filters are complex and expensive.
With increasing use of SAW devices for example in mobile cellular wireless communications systems, and increasing bandwidth choices providable in such systems for example for communications of data and/or telephone signals, it is desirable to provide SAW devices which can facilitate providing switchable or selectable filter bandwidths in a manner that can preserve advantages of SAW devices such as low cost, ease of manufacture, and small size.